I don't know that QTestLib is "better" than one framework for another in such general terms. There is one thing that it does well, and that's provide a good way to test Qt based applications.
You could integrate QTest into your new Google Test based setup. I haven't tried it, but based on how QTestLib is architected, it seems like it would not be too complicated.
Tests written with pure QTestLib have an -xml option that you could use, along with some XSLT transformations to convert to the needed format for a continuous integration server. However, a lot of that depends on which CI server you go with. I would imagine the same applies to GTest.
A single test app per test case never caused a lot of friction for me, but that depends on having a build system that would do a decent job of managing the building and execution of the test cases.
I don't know of anything in Qt Creator that would require a seperate project per test case but it could have changed since the last time I looked at Qt Creator.
I would also suggest sticking with QtCore and staying away from the STL. Using QtCore throughout will make dealing with the GUI bits that require the Qt data types easier. You won't have to worry about converting from one data type to another in that case.
